This invention relates to a tungsten-rhenium filament for high color temperature operation. The invention also relates to a method for manufacturing such a rhenium-tungsten filament, and a halogen incandescent lamp comprising the tungsten-rhenium filament.
Tungsten filaments for incandescent lamps are well known in the art. It is also known that the operation temperature of the filaments determine the light output of the lamp. High operation temperatures of typically between 2900xc2x0 K and 3200xc2x0 K are required for certain applications, as stage- and studio lamps, and special headlamps. However, the lifetime of the filaments tends to decrease dramatically with high operation temperatures. This effect is largely due to the sagging of the filament. Therefore, there is a constant need for improving the non-sag properties of the filaments, particularly at high temperatures.
In order to improve the non-sag property of the filaments, it has been suggested to include small amounts of rhenium in the tungsten. Typically, 1-3% by weight of rhenium is added. E. g. UK Patent No. 1,053,020 teaches the addition of rhenium between 0.1-7% by weight, preferably 3% by weight, in order to improve the mechanical properties of the tungsten. The improvement is accomplished by promoting the formation of elongated grains in the tunsten, as it undergoes a recrystallisation during the lifetime of the lamp. The grain formation is also supported by grain shaping additives, as aluminum, potassium and silicon, commonly known as AKS. The use of such additives is also explained among others in the publication xe2x80x9cThe Metallurgy of Doped/Non Sag Tungstenxe2x80x9d by E. Pink and L. Bartha, published by Elsevier Applied Science, London and New York, 1989.
Further, U.S. Pat. No. 5,072,147 suggests the use of tungsten filaments that are largely recrystallised, and which have a grain structure with elongated interlocking grains. In order to quantify the quality of the grains, it is suggested using the so-called grain shape parameter, which is based partly on the value of the Grain Aspect Ratio (GAR). U.S. Pat. No. 5,072,147 stresses the importance of achieving a large value of the GAR, because it is seen as a key factor for the non-sag property of the filament.
U.S. Pat. No. 6,066,019 also mentions the use of a tungsten-rhenium filament, which is recrystallised before the lamp is actually used. This is necessary because the filament need to be mechanically supported during the recrystallisation. The recrystallisation temperature is above 2600xc2x0 C., i.e. above 2870xc2x0 K.
U.S. Pat. No. 4,413,205 also suggests the use of rhenium for improving the properties of tungsten, but not for improving the grain structure of the filament. Instead, the surface of the integral conductors is improved against the attacks of bromine. The suggested composition contains at least 0.1%, but preferably between 1-3% by weight of rhenium.
While the use of the AKS dopants and the use of rhenium in tungsten is well known for the filaments of incandescent lamps, their use in high color temperature lamps is problematic. The addition of AKS facilitates the grain forming process. However, with increasing color temperatures, particularly above operating temperatures of 2800xc2x0 K, an increased tendency of blister formation on the grain boundaries is observed. These blisters weaken the grain structure, and accelerates the filament degrading process. The formation of the blisters is attributed to the potassium. The addition of rhenium improves the grain structure of the filament, and thereby compensates the negative effect of the potassium, at least partly. It was believed that the addition of at least 1% by weight rhenium is necessary to achieve the desired non-sag properties of filaments operating at high temperatures. It was observed that the grain structure, and thereby the non-sag property improves with higher amounts of rhenium, but even small amounts (as little as 1%) increase the recrystallisation temperature of the tungsten filament above the critical value of 2600-2700xc2x0 K. With presently available mass production technology, the filaments may be heated up to approx. 2750xc2x0 K during the recrystallisation. Raising the recrystallisation temperature above this value would significantly increase the cost of the filament manufacturing.
Therefore, there is a need for a tungsten-rhenium filament with a crystal structure that ensures favorable mechanical properties also at high operating temperatures, and which may be manufactured economically.
In an embodiment of the first aspect of the present invention, there is provided a tungsten-rhenium filament for an operating temperature between 2900-3200xc2x0 K. The filament comprises AKS additive, and has a grain microstructure which comprises substantially exclusively elongated interlocking grains with a Grain Aspect Ratio (GAR) not less than 12. Throughout this description, the term GAR will be used as defined in the U.S. Pat. No. 5,072,147. Further, the filament has a rhenium content of 0.2-0.4% by weight.
In a second aspect of the invention, the method for manufacturing the rhenium-tungsten filament comprises the following steps: An AKS doped tungsten-rhenium alloy powder is prepared, where the alloy powder has a rhenium content of 0.2-0.4% by weight. The alloy powder is pressed and presintered. Thereafter, the alloy powder is sintered with direct current. A filament with a metastable crystal structure is formed of the sintered alloy. The filament is annealed while in the metastable crystal structure, and the annealing is done on a temperature below the recrystallisation temperature. The filament is recrystallised at a temperature above the recrystallisation temperature to achieve a stable crystal structure. This crystal structure has elongated interlocking grains with a GAR not less than 12.
In another embodiment of a further aspect of the invention, the halogen incandescent lamp comprises a glass envelope enclosing a tungsten-rhenium filament. The filament comprises an AKS additive, and has a grain microstructure comprising substantially exclusively elongated interlocking grains with a Grain Aspect Ration (GAR) not less than 12. The rhenium content of the filament is 0.2-0.4% by weight.